The most common way to drive a two-phase stepping motor is by feeding two binary phase non-overlapping electrical signals into the two poles of the motor respectively. The motor will thereby rotate at a certain speed in a single direction. The driving signals are illustrated in FIG. 1. But nevertheless all biased two-phase stepping motors driven in this way will continue its rotation direction when output motion is obstructed. With a preset driving signal in prior art, it is impossible to change the rotation direction under such obstruction conditions. Complicate and costly systems have to be employed to detect the obstruction and change the driving signal such that the motor will rotate in the opposite direction.